In the field of disk storage apparatuses such as hard disk drives (hereinafter simply referred to as disk drives), much effort has recently been made to increase the recording density of disks as storage media. In a disk drive, servo areas (also referred to as servo frames) in which servo data is recorded are arranged on a disk at regular intervals in a circumferential direction. The servo data is position data used to detect the position of a head on the disk. The servo data includes a cylinder number (track number) which identifies the position of a cylinder (track) and a sector number which identifies a sector in the cylinder.
A servo data division method has been proposed in which, for the increased recording density of disks, cylinder codes forming a cylinder number identifying one cylinder are recorded in a plurality of servo areas in the cylinder in a divisional manner. A method has also been proposed in which the sector number is similarly recorded in a plurality of servo areas in a divisional manner. This enables a reduction in the size of each servo area and thus an increase in the size of data areas in which user data is recorded for each cylinder.
According to the servo data division method, during a seek operation without head switching or a normal reproduction (tracking) operation, the current position of the head can be detected by reading the each portions of the cylinder number from the respective plurality of servo areas within the same cylinder and reconfiguring these portions. However, during a seek operation with head switching or a load operation for loading the head from a retracted position onto the disk, the initial position of the head is unknown and an estimated position provided by a state observer is unreliable. This precludes the current position of the head from being accurately detected during a seek operation.